Hydroxyl group-containing vinyl ether polymers are excellent in adhesion with a substrate and processability via a cross-linking reaction with various resins, and are useful as main base agents or blending components for paints, inks, adhesives, resin modifiers, metal recovery resins, compatibilizers, surfactants, dispersing agents, binding agents, or the like.
Since vinyl ether generally has an electron-donating substituent, cationic polymerization is used for production. However, since the hydroxyl group induces a termination reaction for a cationic polymerization catalyst, in cases where a vinyl ether polymer containing the hydroxyl group is obtained by the cationic polymerization, monomers with the hydroxyl group being protected are polymerized and thereafter the protective group needed to be removed by deprotection.
Meanwhile, vinyl ether containing hydroxyl group is shown that radical polymerization is possible since the substituent group acts in an electron-attracting way (refer to Patent Document 1). For example, Patent Document 1 discloses bulk polymerization using 2,2′-azobisisobutyronitrile (AIBN) as an initiator or solution polymerization with water as a solvent, for hydroxyl group containing vinyl ethers such as 2-hydroxyethyl vinyl ether and 4-hydroxybutylvinyl ether. However, monomer conversion rate in polymerization using AIBN is low, and it was not possible to carry out polymerization efficiently.
Patent Document 1 also describes that the monomer conversion rate is improved when a non-nitrile azo based initiator such as dimethyl 2,2′-azobis(2-methylpropionate)(MAIB) is used. It describes that the monomer conversion rate is especially improved in a solution polymerization using an alcoholic solvent such as methanol. However, since hydroxyl group containing vinyl ether exhibits lower radical polymerizability compared with other radical polymerizable monomers, the monomer conversion rate was not improved unless a large amount of polymerization initiator was used under the condition as disclosed in Patent Document 1. Further, even when a large amount of polymerization initiator was used, since the monomer conversion rate is not sufficient, a removal step was required of residual monomers or initiator residues.
Further, it has come to light that even when a non-nitrile azo based initiator and an alcoholic solvent are used, polyacetal is generated depending on the kind of alcohol used as a solvent, resulting in a significant reduction of yield of the hydroxyl group-containing vinyl ether polymer of interest.
Moreover, the monomer conversion rate is further reduced for monomers having many carbons and higher hydrophobicity such as vinyl ether having an alkylene group comprising a cycloaliphatic structure, whereby an efficient polymerization was not possible with the method according to Patent Document 1.